1. Field of the Invention
This invention relates to a method and apparatus for thermal half tone printing, and more particularly a method and apparatus for reproducing a picture image having a half tone produced by making each picture element to correspond to a plurality of dots.
2. Description of the Prior Art
Various methods have been proposed for reproducing a picture image of a half tone with a thermal printing system. According to one method, a picture image of a half tone is printed by making one picture element to correspond to a plurality of dots.
FIGS. 1 and 2 of the accompanying drawing illustrate this method. In each of FIGS. 1 and 2 one picture element is constituted by four fine picture elements comprising four dots. The method shown in FIG. 1 enable to print a picture image havings tones (1) through (9) by combining four fine picture elements that can assume 3 brightnesses of white, black and gray colors. In FIG. 1, white blocks show white color, hatched blocks show gray color and cross-hatched blocks show black color. In FIG. 2, by using black dots having different diameters, printing of 9 tones of (1) through (9) is made possible.
When printing various tones described above, the printing of gray and black colors and the printing of black dots having different size are accomplished by controlling electric energy supplied to respective heat sensitive members comprising thermal heads utilized to print, for example, by controlling the width of the printing pulse. Actually, however, the brightness or size of the printed dots is influenced by heat accumulation of a heating member caused by the heating of a heating member utilized to print a previous line, or by the heating of heating members utilized to print surrounding picture elements so that it has been often impossible to obtain dots having a desired brightness or size.
For example, where a picture element comprising four dots as shown in FIGS. 3a and 3b is to be printed, it is necessary that the brightnesses of the dots shown by a and b should be equal and the sizes of the dots shown by c and d should be equal. However, due to the influence of the accumulated heat caused by the heat generation utilized to print a previous line the brightness of a dot shown by a becomes lower than that of dot b, and the size of the dot c becomes larger than that of dot d.
Heat is accumulated in each picture element due to the fact that even for the same dot, the heating element thereof is influenced by the tone of the nearby picture elements, that is by the heat generated by the heating elements for printing the picture elements. For this reason, it has been impossible to print patterns, letters, etc. having correct brightness and size.
Such phenomenon makes nonuniform the print of half tone, thus making impossible to print desired half tone.